This invention relates to a direct current tight coupling to connect two asynchronous high-voltage three-phase power systems.
To permit power exchange between two systems with differing frequencies (asynchronous systems), one often uses an isolated d.c. circuit in the form of a three-phase--d.c.--three-phase converter between the two power systems. Since the high-voltage d.c. transmission takes place practically at zero distance, this d.c. circuit is described as a "d.c. tight coupling" or a d.c. short coupling.
Such a d.c. tight coupling connecting two asynchronous high-voltage power systems is described in the "Osterreichische Zeitschrift fuer Electrizitaetswirtschaft" (Austrian Journal for the Electrical Industry), volume 36, issue 8/9, pages 265 and following. It comprises primarily two three-phase full-wave static converter assemblies, interconnected on the d.c. side and connected on the three-phase side to both power systems, which, instead of having passive rectifier valves, contain controllable rectifier components (thyristors). This is necessary because the power exchange between the two coupled systems should selectively occur in both directions. For that purpose, the static converter unit allocated to the feeding power system has to function as a rectifier, while the one allocated to the supply network has to function as an inverter. As the mode of operation thus only depends upon the polarity of the static converter voltages, the transmission setup and the extent of the energy flow are determined by the control of the system exclusively and practically independent of the voltage and frequency relationships in the connected three-phase power systems. A prerequisite is the presence of a central thyristor electronic unit, wherein each thyristor is allocated a thyristor electronic subassembly consisting of an output unit to generate and transmit an electronic ignition pulse as well as an information processing electronic component and a signal transfer unit. The signal transfer itself takes place by means of photoconductor or fiber-optic cables for a variety of reasons which, however, are irrelevant to this particular invention.
In the known high-voltage d.c. tight couplings there are always two static converter transformers each, of whose secondary sides one is Wye-connected and the other Delta-connected, operating onto a three-phase full-wave rectifier with twelve semiconductor rectifier components. Each phase of the three secondary three-phase circuits of both static converter transformers is thus allocated two semiconductor components connected in series so that four semiconductor components form one semiconductor subassembly. Each semiconductor component, in turn, consists of two series-connected thyristors. These eight thyristors are generally arranged one atop the other and together form a so-called valve tower. The existing systems thus consist of a total of six valve towers mounted in a so-called valve housing. Each of these valve towers is connected on the a.c. side with one secondary phase connection each of the two allocated static converter transformers and via a d.c. bus each with the two other valve towers allocated to the respective power system.
Since the four static converter transformers are generally mounted on the outside of the valve housing, two each to its two longitudinal sides, and each of the two transformers has to be connected by a line to each of the three valve towers, there is a massive number of high-voltage connecting lines crossing each other, resulting in a correspondingly large spatial requirement. In addition, each of the three adjoining valve towers of one power system has to be connected to the corresponding thyristor electronic unit using the already mentioned fiber-optic cables. For that purpose special high-voltage rated fiber-optic cables are used, whose range of transmission is limited, however. Thus, for example, it is not possible in the known arrangement to mount the transistor electronic unit on one of the frontal sides off the valve housing at the same level as the valve tower and transformers, because that would exceed the maximum range of transmission of the fiber-optic cables. Designers were thus forced to create compartments beneath the valve housing and to mount the thyristor electronic unit within those compartments to assure a short transmission path. Accordingly, it will be appreciated that it would be highly desirable to provide a more compact coupling arrangement, particularly of the semiconductor arrangement.
It is an object of this invention to provide a direct current coupling for connecting two asynchronous high-voltage power systems.
It is also an object of this invention to provide a coupling which has a compact semiconductor arrangement.